1. Field of the Invention
This invention concerns an exposure mask and a method for producing a semiconductor device using the mask and, more in particular, it relates an exposure mask particularly suitable to improve the accuracy of alignment in the exposure step and a method for producing a semiconductor device.
2. Related Art
In the production for a semiconductor device, a circuit pattern for the semiconductor device is formed by transferring a pattern formed on an exposure mask (hereinafter referred to as a mask) on a semiconductor wafer coated with a photoresist (hereinafter referred to as a wafer). The semiconductor device comprises a plurality layers of circuit patterns and exposure of a circuit pattern of a certain layer needs positional alignment with a circuit pattern of a lower layer (alignment).
FIG. 5 is a constitutional view of a step and repeat type exposure system. A wafer 55 is placed on a wafer stage 54 and a reference mark plate 557 is fixed on the wafer stage 54 near the wafer 55. Then, images of a pattern on a mask 512 are projected and exposed by an exposure light from an illumination optical system not illustrated to each of shot regions on the wafer 55 by way of a projection optical system 58. In this case, since the wafer stage 54 is driven along a wafer coordinate system, it is necessary to measure the position at the wafer coordinate system of the mask 512 and angle of rotation of the mask 512 to the wafer coordinate system. For this purpose, two alignment marks (reticule mark) 560R and 561R are formed opposed to each other near the pattern area of the mask 512, and two reference marks 560F and 561F are formed on the reference mark plate 557 at a distance equal with the designed distance for the reticule marks 560R and 561R on the wafer 5.
Further, alignment microscopes 558 and 559 are disposed above the reticule marks 560R and 561R on the mask 512 respectively. Each of the alignment microscopes 558 and 559 has an illumination light source (not illustrated) for emitting an alignment light at the same wavelength as that of the exposure light and a sensor (not illustrated) capable of observing the reticule mark on the mask 512 and the alignment mark on the wafer 55 (wafer mark) or the reference mark on the reference mark plate 557 simultaneously.
When exposure is conducted to the wafer 55 by the exposure device shown in FIG. 5, only the wafer stage 54 is moved in a step and repeat manner and images of a pattern on the mask 512 are exposed to each of shot regions of the wafer 55 respectively. In the exposure device described above, when a pattern image of the mask 512 is exposed further on the circuit pattern on the wafer 55 formed in the preceding step, it is necessary to correspond the wafer coordinate system defining the coordinate for each of the shot regions on the wafer 55 with the mask coordinate system defining the coordinate system for the pattern on the mask 512, that is, to take alignment.
The alignment is conducted, for example, by the following procedures. At first, after driving the wafer stage 54 to move the reticule mark plate 557 into an exposure field of the projection optical system 58, the amount of the positional displacement between the reticule mark 560R and the reference mark 561F is detected by one reticule alignment microscope 558 and determine the pattern position for the reticule 512 on the wafer coordinate system based on the amount of the positional displacement. Further, the wafer stage 54 is driven to move the reference mark 560F to the position for the reference mark 561F to detect the amount of positional displacement between the reticule mark 561R and the reference mark 560F by the reticule alignment microscope 559, to measure the angle of rotation of the reticule 512 in the wafer coordinate system. Then, the reticule 512 or the wafer stage 514 is rotated to correct the angle of rotation and correspond the wafer coordinate system with the reticule coordinate system.
Further, in FIG. 5, an off axis system alignment microscope 534 is disposed on the lateral side of the projection optical system 58 in order to detect the position for the wafer mark formed corresponding to each of the shot regions on the wafer 55. In this case, based on the position for the wafer mark detected by the alignment microscope 534, the corresponding shot region on the wafer 55 is set within the exposure region of the projection optical system 58. Accordingly, it is necessary to previously determine the amount of a base line that is a distance between the reference point in the exposure field (for example, center for exposure) of the projection optical system 58 and the reference point 62 in the observation region of the off axis system alignment microscope 534.
When the amount of the base lines is measured, the amount of positional displacement of conjugate images between the reticule marks 560R and 561R, and the reference marks 560F and 561F on the wafer stage 54 are measured and then, the wafer stage 54 is moved, for example, by an amount equal with the designed value for the amount of the base line, the amount of positional displacement between the reference point 562 and the corresponding reference mark on the reference mark plate 557 is measured by the alignment microscope 534, to determine the amount of the base line based on the amount of the positional displacement thereof.
Since each of the operations for alignment explained above is conducted by using the alignment mark formed on the mask, if the position for the alignment mark on the mask includes an error, it forms an alignment error of the wafer to be exposed as it is (alignment error between the pattern on the wafer formed in the succeeding step and the pattern on the wafer to be formed in the succeeding step), to possibly lower the yield of the semiconductor device.
Particularly, rapid development for the refinement of semiconductor devices in recent years, has brought about a problem that the error of the pattern position on the mask caused by mask drawing gives an undesired effect on the alignment accuracy upon exposure and inventions for the exposing method to reduce the effect have been proposed.
For example, Japanese Published Unexamined Patent Application Hei 7-176468 (Related Art 1) discloses an example of a scanning type projection exposure method of using a mask on which a plurality of alignment marks are formed in a scanning direction, measuring the positions for the marks on an exposure device and reducing the effect of the mask drawing error by the averaging effect for the result of measurement.
Further, Japanese Published Unexamined Patent Application Hei 7-29803 (Related Art 2) discloses an example of an exposure device adapted to determine the mask drawing error based on the measurement of the positions for a plurality of patterns on a mask by an exposure device and conduct scanning exposure while moving the mask or the wafer so as to correct the error upon exposure.
The prior arts 1 and 2 reduce the error by averaging the overall drawing errors of the mask but no consideration is taken on the relative positional error between the reticule alignment mark and the device area. Accordingly, they are not always satisfactory for the correction of the alignment error caused by reticule alignment.
Further, while the prior arts 1 and 2 measure the accuracy for the drawing position of the mask on the exposure device, since the exposure device has to be used for the operation other than exposure, this causes lowering of the throughput in the device production to bring about a problem, particularly, in multiplicity kind small lot production such as in system LSI using plural kinds and a number of reticules.
This invention provides a technique capable of solving the subject in the prior art described above and obtaining a desired alignment accuracy in a device area, even when the there is a relative positional error between the alignment mark on the mask and the device area, by measuring a relative positional error between alignment marks and a device area on the mask and setting the result of the measurement as a correction value upon exposure in the exposure device.
That is, in the exposure mask according to this invention, a relative positional displacement between the alignment marks and a device area formed on a substrate is measured and the data is appended as quality guarantee data.
Further, in the exposure mask according to this invention, measurement for the relative positional displacement between the alignment marks and the device area on the substrate is measurement for relative position between the alignment marks and marks for the measurement of alignment disposed on four corners of a device area respectively.
Further, in the exposure mask according to this invention, the result of measurement for the relative positional displacement between the alignment marks and the device area is on the substrate expressed by the amount of rotation and the amount of shifting of the device area to the alignment marks. This enables to facilitate the production for a semiconductor device according to this invention shown above.
Further, a method for producing a semiconductor device according to this invention comprises measuring a relative positional displacement between an alignment mark and a device area on an exposure mask and correcting alignment by using the result of measurement upon exposure.
Further, in the method for producing a semiconductor device according to this invention, measurement for relative positional displacement between the alignment and the device area on the exposure mask is measurement for the relative position between the alignment marks and the marks for measurement of alignment disposed respectively at four corners of the device area.
Further, in the method for producing a semiconductor device according to this invention, measurement for relative positional displacement between the result of the alignment marks and the device area on the exposure mask is expressed by the amount of rotation and the amount of shifting of the device area to the alignment marks. This enables to facilitate feedback to the alignment correction upon exposure.
Further, in the method for producing a semiconductor device according to this invention, measurement for relative positional displacement between the alignment marks and the device area on the exposure mask is conducted in an off line manner by using a device other than the exposure device. Since the measurement is conducted in the off line manner, alignment accuracy can be improved giving no undesired effect on the throughput in the exposure step.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.